In a so-called multi-air-conditioning apparatus, a plurality of indoor units can be connected to a single outdoor unit. Therefore, the multi-air-conditioning apparatus is capable of air-conditioning a plurality of rooms even in an apartment having a limited space for installation of the outdoor unit. The above-mentioned multi-air-conditioning apparatus enables space-saving, has high exterior appearance design quality, and is advantageous in terms of costs. Thus, in recent years, the multi-air-conditioning apparatus has been more widely used.
In general, if a heating operation is continued by an air-conditioning apparatus, an outdoor heat exchanger, which functions as an evaporator, is frosted. The frost formation on the outdoor heat exchanger becomes a factor of degradation of heat exchange efficiency, resulting in lowered heating performance. In the case of the multi-air-conditioning apparatus, the plurality of indoor units can be connected to the single outdoor unit. Therefore, when the heating operating is performed, the amount of operation of a condenser is likely to relatively increase. As a result, the multi-air-conditioning apparatus has a problem in that an evaporating pressure of the evaporator is likely to be lowered to more easily form frost.
In order to improve the heating performance that is lowered during the operation, a defrosting operation for melting the frost adhering to the outdoor heat exchanger is performed. The defrosting operation is performed by switching a four-way valve to switch between a heating circuit and a cooling circuit. The cooling circuit functions during the defrosting operation. Therefore, there is a problem in that hot refrigerant does not flow through indoor heat exchangers to result in a lowered room temperature. Therefore, the defrosting operation is desired to be performed at an appropriate timing.
In an ultralow-temperature area, for example, an area in which the temperature becomes −7 degrees Celsius or lower, the humidity does not become high except for under special environments. Therefore, the outdoor heat exchanged is unlikely to be frosted. In a case where the outdoor unit does not include a humidity sensor, the outdoor air humidity cannot be measured. Thus, the defrosting operation is sometimes unnecessarily performed even when the outdoor heat exchanger is not frosted.
Hitherto, there is proposed a freezing detection device for an evaporator of a refrigeration device, which is configured to measure a discharged refrigerant temperature and detect freezing of an evaporator when the degree of decrease in discharged refrigerant temperature becomes equal to or larger than a predetermined value (see, for example, Patent Literature 1).
Besides, there is proposed an air-conditioning apparatus, which is configured to measure a condensing temperature of a condenser and allow the defrosting operation to be performed when a state, in which a temporal change amount of the condensing temperature is equal to or larger than a predetermined value, lasts for a predetermined set time period or longer (see, for example, Patent Literature 2).